The present invention relates to improved bearings for use in orthopaedic implant prostheses and particularly to methods for making cross-linked molded ultra high molecular weight polyethylene (UHMWPE) bearings and bearing material.
Such UHMWPE resin is commonly used for bearings in hip, knee, shoulder and elbow prosthesis. Typically, the bearings may be formed by direct compression molding processes or by machining the required bearing shapes from mill shapes such as sheet or bar stock material. Typically, the stock material or the molded bearings are irradiated and subsequently heat treated or heat annealed. The irradiation generates molecular cross-links and free radicals. The free radicals are subsequently eliminated by the heat treating processes.
Reference is made to a number of prior art references as follows:
U.S. Patents:
1. U.S. Pat. No. 5,414,049, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
2. U.S. Pat. No. 5,449,745, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
3. U.S. Pat. No. 5,543,471, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
4. U.S. Pat. No. 5,650,485, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
5. U.S. Pat. No. 5,728,748, Non-Oxidizing Polymeric Medical Implant, to Deh-Chuan Sun, et al.
6. U.S. Pat. No. 4,586,995, Polymer and Irradiation Treatment Method, to James C. Randall.
7. U.S. Pat. No. 5,153,039, High Density Polyethylene Article with Oxygen Barrier Properties, to Jay P. Porter, et al.
8. U.S. Pat. No. 5,508,319, High Melt Strength, Ethylene Polymer, Process for Making It, and Use Thereof, to Anthony J. DeNicola, Jr. et al.
9. U.S. Pat. No. 3,352,818, Stability of Polyolefines, to Gerhard Meyer, et al.
10. U.S. Pat. No. 5,577,368, Method for Improving Wear Resistance of Polymeric Bioimplantable Components, to John V. Hamilton, et al.
11. U.S. Pat. No. 5,753,182, Method for Reducing the Number of Free Radicals Present in Ultrahigh Molecular Weight Polyethylene Orthopedic Components, to Joel Higgins.
12. U.S. Pat. No. 5,709,020, Method for Reducing the Generation of Wear Particulates From an Implant, to David A. Pienowski, et al.
13. U.S. Pat. No. 5,702,456, Implant Having Reduced Generation of Wear Particulates, to David A. Pienowski.
14. U.S. Pat. No. 5,515,590, Method for Reducing the Generation of Wear Particulates From an Implant, David A. Pienowski.
15. U.S. Pat. No. 5,593,719, Treatments to Reduce Frictional Wear Between Components Made of Ultra-High Molecular Weight Polyethylene and Metal Alloys, Geoffrey Dearnaley, et al.
16. U.S. Pat. No. 4,366,618, Bone Connective Prosthesis Adapted to Maximize Strength and Durability of Prostheses xe2x80x94Bone Cement Interface, and Methods of Forming Same, to Simon Raab.
17. U.S. Pat. No. 5,014,494, Method of Sterilizing Medical Articles, to Robert D. George.
18. U.S. Pat. No. 5,137,688, Irradiated Articles Molded From Polycarbonatexe2x80x94Polyamide Blends, to James L. DeRudder.
19. U.S. application Ser. No. 08/911,792, Process for Medical Implant of Cross-linked Ultrahigh Molecular Weight Polyethylene Having Improved Balance of Wear Properties and Oxidation, filed Aug. 15, 1997, to Kenneth Ashley Saum, et al.
Foreign Patents
20. E.P. Patent No. 0722973 A1, Chemically Crosslinked Ultrahigh Molecular Weight Polyethylene for Artificial Human Joints, to Ron Salovey, et al.
21. W.O. Patent No. 97/29793, Radiation and Melt Treated Ultra High Molecular Weight Polyethylene Prosthetic Devices, to W. Merrill, et al.
22. W.O. Patent No. 98/01085, Crosslinking of Polyethylene for Low Wear Using Radiation and Thermal Treatments, to Fu-Wen Shen, et al.
The above references teach the general concepts involved in forming UHMWPE resin directly into a part or a stock form from which the part is made, gamma or other irradiation of the part or the stock form and subsequent heat treating (annealing or remelting) of the part or stock form. The disclosures of these above-listed references are incorporated herein for purposes of establishing the nature of UHMWPE resin, the irradiation steps and options and the heat treating steps and options.
The present invention is directed to a method for irradiating the UHMWPE resin when it is in powder form, which produces cross linking of the molecular chains as well as free radicals. The free radicals generated from the gamma irradiation are quenched by the subsequent molding process.
While the prior art teaches irradiating formed bearing parts or stock from which bearings are made, the present invention involves irradiation of the UHMWPE resin powder, preferably by gamma or electron beam irradiation. This ionizing irradiation will generate molecular cross-links and free radicals as is well documented. These free radicals will be dealt with in the present invention by the molding process which involves temperatures above the melting point of the polymer.
The present invention is designed to reduce the amount of wear debris generated from UHMWPE bearings. Such wear debris has been associated with bone and soft tissue deterioration leading to implant loosening, which may necessitate even revision surgery. The present invention is also to improve the wear resistance of UHMWPE bearings.
Additional features of the present invention will become apparent to those skilled in the art upon consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.
The present invention involves producing a cross-linked bearing component by cross-linking the UHMWPE resin powder and then using the thermal energy of the molding process to promote further cross-linking by free-radical quenching. The process of the present invention may be used to mold net-shape bearings directly to provide a finish which is satisfactory for an articular (bearing) surface. (The term xe2x80x9cnet-shapexe2x80x9d is intended to define a shape that is the final shape of the bearing to be used in the implant. xe2x80x9cNear net-shapexe2x80x9d requires some degree of machining to produce the final bearing.) It is believed that smoother bearing surfaces can be made by molding processes than by machining processes.
The irradiation process may preferably be performed under vacuum, in an inert atmosphere to eliminate oxygen, or in an oxygen reduced atmosphere. Oxygen present when irradiating has been shown to cause subsequent reduction in mechanical properties for UHMWPE. A premolding vacuum phase is also contemplated by the present invention to enhance removal or reduction of any unwanted oxygen from the UHMWPE resin powder. For example, the irradiated powder resin may be cold pressed into the desired shape and then left under vacuum before heat is applied.
The molding process, which may also preferably be performed under vacuum, may involve pressures from 1,000 to 70,000 psi and set point temperatures of 250xc2x0 to 500xc2x0 Fahrenheit. The resulting work piece is either the actual desired bearing configuration or blank that can be machined subsequently to produce the desired bearing geometry.
It will be appreciated, however, that acceptable results may be achieved when either or both the irradiation process and the molding process are carried out under atmospheric conditions, i.e., with some oxygen present. This may well be the case for certain bearing configurations.
One embodiment of the present invention relates to a method for producing a medical implant or stock from which an implant can be made from olefinic material such as UHMWPE comprising the steps of placing olefinic resin in a sealed container, preferably evacuating or displacing air from the container to produce a low oxygen or oxygen free atmosphere, subjecting the resin container to irradiation, preferably gamma or electron beam irradiation, at a dose level between 2 and 50 Mrads to create cross-links and free radicals within the polymer resin, preferably maintaining an inert atmosphere, and subjecting the free radical containing powder to a molding operation which preferably takes place in a vacuum or in the inert atmosphere. The irradiated resin may preferably be transferred from the scaled container to the molding equipment in a vacuum or in an inert atmosphere environment or may be exposed to air for a short time during the transfer.
The molding operation may comprise the steps of raising the powder resin within the mold to a temperature at or above the melting point of the resin. The purpose of this is to produce a consolidated net-shape or near net-shape implant or raw material (bar, sheet or puck stock) while simultaneously quenching the free radicals to promote further cross-linking between molecules of the polymer.
The sealed container may be flushed with inert gas (nitrogen, argon, helium, neon, or any combination thereof) and evacuated several times to further reduce the residual oxygen content. Instead of a vacuum, the atmosphere of the scaled container prior to gamma irradiation and storage may also be an inert gas consisting of nitrogen, argon, helium, neon, or any combination thereof.
A more complete understanding of the present invention can be obtained by referring to the following illustrative examples of the practice of the invention, which examples are not intended, however, to be unduly limitative of the invention.